A Preliminary Economic Assessment describes a dual-circuit gold recovery design combining static and recirculating vat leaching for higher-grade feed with heap leaching for lower-grade feed, based on historical operating precedent and preliminary testwork.
Report context
This Preliminary Economic Assessment (PEA) for the Gold Rock Project, located in White Pine County, Nevada, USA, is dated March 31, 2020. The report presents a conceptual process design intended to minimise capital expenditure over a relatively short mine life based on the current mineral resource estimate, while maintaining gold recovery.
Processing route
Higher-grade process circuit
For process feed above 0.015 opt Au, a vat leaching circuit is proposed. The design incorporates primary, secondary and tertiary crushing stages followed by open-circuit rod milling to a P80 of 28 mesh. A cycloning step separates the mill product: the 28 mesh X 150 mesh fraction reports to cyclone underflow for static sand vat leaching, while the nominal -150 mesh fraction reports to cyclone overflow for recirculating vat leaching.
The static sand vat circuit is designed to consist of seven vats, with one filling, one unloading, and five under leach at any given time. Each vat has a capacity of 7,000 st, and the design treats 4,800 stpd of sand fraction over a seven-day leach cycle. Pregnant solution from the sand vats is pumped to a carbon-in-column (CIC) carousel circuit at a rate of 750 to 1,000 usgpm.
The recirculating slimes vat circuit is designed to consist of four vats, with one filling, one unloading, and two under leach at any given time. Each vat has a capacity of 4,000 st, and the design treats 1,200 stpd of slimes material at 25% solids with two days of leach time. The slimes slurry is recirculated through a carbon-in-pulp (CIP) circuit. The recirculation pumping system is designed to turn over the entire vat volume every eight hours, with open-to-atmosphere discharge providing aeration.
Lower-grade process circuit
For material below 0.015 opt Au but above 0.004 opt Au, heap leaching is planned. The low-grade crushing circuit uses an open-circuit horizontal shaft impact crusher (HSI) to produce a P80 of 3 inches at up to 6,000 stpd. Following crushing, the product is to be belt-agglomerated with dewatered vat tailings and cement prior to stacking.
Carbon and gold recovery circuits
A single six-stage CIC carousel treats pregnant solution from both the sand vat leach circuit and the heap leach circuit. A separate six-stage CIP circuit, configured as two trains of three stages each operated in series, recovers gold from the slimes vat leach slurry.
Loaded carbon from both CIC and CIP circuits is fed to a common pressure Zadra style elution system, including acid treatment and carbon regeneration. A horizontal kiln carbon regeneration circuit is assumed for this PEA. Gold is recovered from strip solution by electrowinning onto steel wool cathodes, with further retorting to remove mercury, followed by smelting to produce doré bars.
Tailings integration
Spent vat tailings from both sand and slimes vats are removed hydraulically, dewatered via cyclone and dewatering screen to approximately 20% contained moisture, then belt-agglomerated with crusher-run heap leach material and cement before placement on the heap. No tailings storage facility is required under this conceptual design.
Key reported parameters
| Parameter | Basis | Value |
|---|---|---|
| Vat circuit design feed rate | Design | 6,000 stpd |
| Heap leach circuit average feed rate | Design (preliminary mine planning) | 3,700 stpd |
| Heap leach circuit feed rate range | Design (preliminary mine planning) | 2,800 to >5,800 stpd |
| Total design feed rate (both circuits) | Design | 10,000 stpd |
| Crushing circuit design product (vat feed) | Design | P80 3/8 inch |
| Crushing circuit maximum capacity | Design | 10,000 stpd to P80 5/8 inch |
| Rod mill design product | Design | P80 28 mesh (open circuit) |
| Rod mill nominal specification | Estimate (no Bond work index testing) | 12 ft x 20 ft, +/- 950 hp |
| Sand vat leach retention time | Design | 7 days |
| Sand vat leach capacity per vat | Design | 7,000 st |
| Slimes vat leach retention time | Design | 2 days |
| Slimes vat leach capacity per vat | Design | 4,000 st |
| Combined vat recovery (static + recirculating) | Estimated (preliminary testwork) | 90% |
| Net gold recovery after CIC, acid treatment, EW, smelting | Estimated (vat circuit) | 88.2% |
| Net gold recovery from heap leach | Estimated (cyanide soluble gold determinations) | 60% |
| CIC preg solution feed rate range | Design | 750 to 1,000 gpm |
| Carbon in pulp slurry concentration | Design | 1.5 lb/ft³ per stage |
| Carbon in pulp stage carbon capacity | Design | 2 tons per stage |
| Carbon in column carbon capacity per column | Design | 2 tons per column |
| Carbon in column column dimensions | Design | 7 ft diameter X 7 ft height |
| Low-grade crushing product size | Design | P80 3 inch |
| Cyanide consumption (heap leach) | Estimated (factored from similar projects) | 0.25 lb/ton NaCN |
| Cyanide consumption (sand + slimes vat leach) | Estimated (factored from similar projects) | 1.0 lb/ton NaCN |
| Lime consumption (heap leach) | Estimated (factored from similar projects) | 3.0 lb/ton CaCO₃ |
| Lime consumption (sand + slimes vat leach) | Estimated (factored from similar projects) | 4.0 lb/ton CaCO₃ |
Project website: https://drydengold.com/project/gold-rock-project/
Technical qualifications
The report notes that additional detailed metallurgical testwork will be required to confirm that Gold Rock mineralisation is amenable to economic vat leaching, and that this element constitutes some risk to project economics.
The vat leach recovery estimate is based on preliminary metallurgical testwork discussed in Chapter 13 of the report, not on demonstration-scale testwork on Gold Rock material. The report states that combined vat recovery is estimated at 90%, with 88.2% net gold recovered for sale after downstream processing.
Reagent consumptions are estimated by factoring based on review of available test results and experience with similar projects; no site-specific testwork results for reagent consumption are reported.
Bond work index testing for the rod mill design had not been performed at the time of the report; the rod mill specification is based on typical expectations for the project.
The report notes that the vat process design was successfully operated at the Homestake Gold Mine for over 20 years, achieving recoveries for higher-grade zones of approximately 90%. This historical data is cited as operational precedent, not as testwork on Gold Rock material.
Heap leach recovery is based on cyanide soluble gold determinations from recently completed core drilling, not on column leach testwork.
*Source: Gold Rock Project , 2020 Technical Report (PEA), March 31, 2020, sections: Recovery Methods, Processing Plant and Facilities, Higher-Grade Process Circuit, Lower-Grade Process Circuit, Activated Carbon Circuit, Electrowinning, Retorting and Smelting Circuit.*
